A multiplicity of micromechanical actors and sensors are based on the utilization of electrostatic forces for achieving deflection of a functional structure and/or on capacitive methods for determining a deflection of the functional structure. In many cases, deflection of a body, which is movably suspended via one or more springs, is necessary for accomplishing the functionality of the actor and/or sensor, wherein the deflection of the body should be possible at least in two dimensions. In general, translatory and/or rotatory movements, i.e. rotational movements, are possible. In order to achieve this, various static electrodes, which can be contacted electrically in independent manner from each other, may be arranged below the movable body, so that the body, depending on between which static electrode and the movable body an electric voltage is applied, is deflected in the direction of the respective static electrode and/or the externally induced motion of the body can be detected capacitively.
Such an arrangement is described in U.S. Pat. No. 7,078,778 B2. What is disadvantageous in such an arrangement is the relatively complex construction as well as the connection technology of the arrangement. In general, such devices may also be tested only after a dicing process of the device, which leads to faulty devices being discovered relatively late in the fabrication process, which may be connected to significant costs. The above-mentioned arrangement further necessitates relatively high electric voltages, and geometric restrictions result for the deflection angle of the deflectable body due to the construction. With too large deflections and/or with external disturbances, e.g. by oscillations or external force impact (shock), there is the risk of the so-called pull-in, which may result in undesired deflection of the electrode arrangement, which may even lead to mutual contact of the electrode arrangement, leading to the destruction of the micromechanical device in the extreme case.
With the aid of comb electrodes or electrode combs, which comprise a multiplicity of so-called fingers, relatively great forces and/or moments can be generated. This is due to the fact that the distance of the comb electrodes typically is two to ten micrometers, and thus relatively small. At the same time, a relatively high capacitive change on movement of the electrode combs against each other can be taken advantage of through the comb-shaped arrangement of the electrode fingers, which interdigitate without touching in the normal case of operation.
In EP 1 123 526 A, there is described an arrangement in which movement out of the chip plane is to be generated. What is described there is an arrangement in which fixed electrode combs and movable electrode combs are produced in a layer and are not juxtaposed with respect to each other in offset manner. In this construction, static deflection of the suspended body cannot be achieved. Rather, this approach can only be used for resonantly operated devices, such as resonantly operated scanner mirrors. In resonantly operated scanner mirrors, the scanner mirror is energized by applying a suitable time-dependent voltage of a certain frequency in periodic intervals for maintaining the oscillation of the scanner mirror.
A series of publications describe scanner mirrors in which the comb electrodes are arranged in offset manner with respect to each other. Fixed electrode combs are connected to the chip frame. Movable combs are connected either to the mirror plate, or the micromechanical functional structure in general, or directly to a spring, which holds the structure. In parts, the movable combs are also mounted to additional beams or other structural elements, which in turn are connected to the mirror plate, which represents the deflectable suspended body, or to the spring.
In U.S. Pat. No. 6,891,650 B2, there is described a scanner mirror in which the fixed and movable electrode combs are produced in different layers. The two layers are separated from each other by insulating material, e.g. an insulation layer. By way of the construction, the two electrode combs are arranged in parallel and perpendicularly to the chip surface in shifted manner with respect to each other. Upon application of a voltage, the movable comb may now be drawn in the direction of the fixed electrode comb. In a suspension of the scanner mirror via a torsion spring, this takes place in form of a tilt, until an electrostatic moment, which is induced by an electric voltage between the capacities formed by the two electrode combs, and a mechanical restoring moment, which is induced by the torsion spring, are in balance. What is disadvantageous here, apart from the complex processing, which is performed with a so-called deep reactive ion etching (DRIE) multiple etch, above all is the fact that the capacity change, to which the electrostatic moment is directly proportional, decreases particularly with great deflections. Moreover, the construction no longer is symmetrical in a tilt of the mirror plate. This results in the fact that the electrostatic forces are clearly greater in a tilt on the one side of the mirror plate than in a tilt of the mirror plate to the other side. Hence, the maximum electric voltage applicable, and hence the deflection, is restricted, because the above-mentioned pull-in effect may occur at too high a voltage. A similar construction is described in EP 1,659,437 A2.
In JP 2004-219839 A, there is described a construction in which the movable electrodes are not shifted in parallel to the fixed electrodes, but are arranged in tilted manner with respect thereto. To this end, the fixed electrode comb is suspended via a torsion spring in parallel to a torsion axis of the mirror, and may be tilted thereabout. What is disadvantageous in this construction is that the construction no longer is symmetrical when tilting the mirror plate. The electrostatic forces are clearly greater in the tilt on the one side of the mirror plate than in a tilt to the other side. Thereby, the maximum electrical voltage applicable, and hence the deflection, is restricted, because the so-called pull-in effect may occur at too high a voltage between the electrode combs.
J. Kim et al. (Microfabricated Torsional Actuators using self-aligned plastic deformation of silicon”, J. Micromechanial Systems, vol. 15, no. 3, June 2006, p. 553 ff) present an approach in which the mirror plate is deflected temporarily via a mandrel. In parallel to the torsion axis of the mirror axis, there is a structure with the movable electrode combs, so that these are tilted with the mirror plate. The construction is then subjected to high-temperature treatment. Here, the torsion spring, which is under mechanical stress, deforms plastically, so that the deflected state remains when cooling also after removal of the mandrel. What is disadvantageous in this variant, however, is the processing. The high temperatures may for example lead to destruction and/or decomposition of aluminum-based conductive traces or also aluminum-based mirrorings. Furthermore, it is to be assumed that the high temperature stress may have negative effects on the curvature of a mirrored plate. In addition, this construction also distinguishes itself by the above-described asymmetry, which may lead to early pull-in of the electrodes.
By D Hah et al. (“Theory and Experiments of Angular Vertical Comp-Drive Actuators for Scanning Micromirros”, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, vol. 10, no. 3, May/June 2004, p. 505 f), there is presented an approach in which the movable combs, which are connected to the torsion spring of the scanner mirror via a polymer, e.g. lacquer or polychlorinated biphenyl (PCB) polymer, are tilted. To this end, a so-called reflow process for the polymer is used, consisting of a temperature treatment in combination with a chemical treatment. In this process, the movable combs, which are connected to the torsion spring of the scanner only via the polymer hinge, are deflected. What may be disadvantageous here is the use of polymers, with respect to the reliability. In particular, varying humidity, radiation of light, and aging may lead to failure of the hinges and to a significant temperature dependency of the tilting angle. In addition, this construction also distinguishes itself by the above-described asymmetry, which may lead to early pull-in.
Apart from the electrostatic drives, there exist a series of devices based on piezoelectric, thermal or magnetic drives. Piezoelectric drives are more difficult to integrate than electrostatic drives. Particularly when complementary metal oxide semiconductor (CMOS) process compatibility is necessary, the choice of possible materials becomes limited. Other efficient piezoelectric materials, such as lead zirconate titanate (PZT), have spontaneous polarization decreasing with time. For this reason, renewed polarization must take place at elevated temperatures at certain time intervals. For many applications, this is not feasible. Other materials, such as aluminum nitride (AlN), indeed have spontaneous polarization invariable over time and CMOS compatibility, but possess significantly smaller piezoelectric constants than PZT. Hence, the efficiency is not sufficient in many cases.
Thermals drives, which are based on the bimorph effect, per se have a dependence of the drive on the ambient temperature. This must than additionally be compensated for via regulation. For applications that have to cover a large temperature range, such as in the automotive area, these actors generally are not suited.
Magnetic drives have sufficiently high efficiency (for example Bimag by Microvision). The device as such, however, is significantly more complex than in the case of an electrostatic drive. While all components can be integrated in the electrostatic drive, an external magnet is employed in the magnetic drive. Not only is this cost-intensive, but also has to be accurately aligned, and further significantly enlarges the overall construction. Moreover, such a magnetic drive may have comparably high power consumption.